U.S. DEFENSE SECRETARY JIM MATTIS: Any threat to the United States or its territories, including Guam, or our allies will be met with a massive military response, a response both effective and overwhelming.

KELLY: Pyongyang says the bomb tested yesterday was a hydrogen bomb. That has not been confirmed, but it does look like this one was much more powerful than any bomb North Korea has tested so far. NPR's Elise Hu is following this story from across the border in Seoul, South Korea. She's on the line now. Hey, Elise.

ELISE HU, BYLINE: Hey there.

KELLY: How does this weekend's test change the stakes in this crisis?

HU: Well, it does and it doesn't. It doesn't change things in that all of us in this region were fully expecting the sixth test to happen at some point. North Korea has been consistently trying to test and improve its program, and it has to test in order to improve. And the regional actors here, as well as the U.S., really haven't changed and diverted from the typical playbook between last year's test in September and this recent one. And so without a dramatic new move, the course didn't change. Now, this could...

KELLY: So that's how it didn't change. How does the course change by all this?

HU: (Laughter). That's right.

KELLY: Yeah.

HU: Well, it does worsen the situation if regional players are reading the fast advances of the North Korean program and then decide to act differently. And what we mean by differently is it could embolden those in Japan who want to build up militarily, conventional arms even. Or does it drive the U.S. into taking more seriously into consideration military options? And that's despite all the catastrophic collateral damage that it could trigger.

KELLY: Sure. So that's what could happen on the military front. Now, what about on the economic front? Recently the U.N. passed new sanctions, sanctions that do not appear to be reigning in North Korea's nuclear ambitions one bit. What else might world leaders do at this point to address the situation?

HU: Well, even despite the spotty enforcement of sanctions and the poor record so far, we are likely to see more pressure on Pyongyang in the form of even more sanctions. But at the same time, there's going to be more pressure in the form of the saber-rattling and shows of force and just more talk of military options, as we heard from Jim Mattis previously. So there was already a show of force earlier today here on the peninsula in the form of surface-to-surface missiles launched by South Korea.

KELLY: OK.

HU: Japan is talking about what else it can do to isolate North Korea unilaterally. And what we're not hearing about, though, is any talk about direct negotiations with Pyongyang, which would be a diplomatic, nonmilitary option to confront this crisis. President Trump has tweeted recently that talking is not the answer. However, South Korea would be most interested in finding a peaceful solution considering our proximity here to the north.

KELLY: Speaking of South Korea, President Trump seemed to take aim at South Korea. He was tweeting following this nuclear test and saying South Korea should not try to appease North Korea. Why criticize South Korea, and why now?

HU: Yeah. It's unclear, and it's not being received well in Seoul, where the national security council here has already sought clarification from Washington on that tweet. And when you want partners in isolating North Korea, alienating a key longtime ally seems like a strange way to go about things.

KELLY: That's NPR's Elise Hu talking to us there from Seoul. Thanks, Elise.